Chemical-mechanical polishing ("CMP") processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a relatively soft, porous material such as blown polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. An under-pad 25 is typically attached to the upper surface 22 of platen 20, and the polishing pad 40 is positioned on the under-pad 25. A drive assembly 26 rotates the platen 20 as indicated by arrow A, or in another existing CMP machine the drive assembly 26 reciprocates the platen 20 back and forth as indicated by arrow B. The motion of the platen 20 is imparted to the pad 40 through the under-pad 25 because the polishing pad 40 frictionally engages the under-pad 25. The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows C and D, respectively.
In the operation of the conventional planarizer 10, the wafer 12 is positioned face-downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 12 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 12.
In the competitive semiconductor industry, it is desirable to maximize the throughput of the finished wafers and minimize the number of defective or impaired devices on each wafer. The throughput of CMP processes is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate"). Because the polishing period per wafer decreases with increasing polishing rates, it is desirable to maximize the polishing rate within controlled limits to increase the number of finished wafers that are produced in a given period of time.
One problem with CMP processing is that the throughput may drop because the condition of the polishing surface on the pad deteriorates while polishing a wafer. The deterioration of the polishing pad surface is caused by waste particles from the wafer, pad, and slurry that accumulate on the polishing surface of the polishing pad. Since the accumulations of waste particles alter the condition of the polishing surface on the polishing pad, the polishing rate tends to drift over time. For example, after polishing a single wafer for only 4 minutes with a Rodel IC-1000 polishing pad and a Rodel ILD-1300 slurry (both of which are manufactured by Rodel Corp. of Arizona), the polishing rate drops and reduces the throughput for semiconductor wafers. Many semiconductor manufacturers accordingly recondition the pad after each wafer, but unless the reconditioning process is performed in situ and in real-time, then the reconditioning of the pad also causes down-time that reduces throughput. Thus, the waste particles on the polishing surface reduce the throughput of the CMP process.
CMP processes must also consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus the image of circuit patterns on the surface of the wafer. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the circuit pattern to better than a tolerance of approximately 0.1 .mu.m. Focusing the circuit patterns to such small tolerances, however, is very difficult when the distance between the emission source and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, several devices may be defective on a wafer with a non-uniformnly planar surface. Thus, CMP processes must create a highly uniform, planar surface.
Another problem with CMP processing is that the accumulations of waste particles reduce the uniformity of the polished surface because they do not accumulate uniformly across the polishing surface of the pad. The polishing rate accordingly varies from one region on the pad to another resulting in a nonuniform polished surface on the wafer. Therefore, in light of the problems associated with accumulations of waste particles on polishing pads, it is necessary to periodically clean and condition the polishing surface to remove such accumulations and bring the polishing pad back to a desired condition.
Polishing surfaces on polishing pads are typically cleaned by brushing the pad with a brush, or by flushing the pad with a fluid. To perform the brushing and flushing processes, the wafer is generally removed from the pad while the brush or fluid engages the polishing surface of the polishing pad. One problem with the brushing and flushing processes, therefore, is that a significant amount of down-time is necessary to merely clean the polishing pad. Thus, it would be desirable to develop a pad cleaner that effectively cleans the pad in situ and in real-time.
Polishing surfaces of polishing pads are typically conditioned with a diamond embedded plate mounted to a separate device that moves the plate across the polishing pad to abrade the surface of the pad. Some pad conditioners remove a portion of the upper layer of the deteriorated polishing surface to form a new, clean polishing surface. One problem with conventional pad conditioners is that they condition the pad substantially uniformly across the polishing surface. Since the wafers only polish on a well-defined area of the pad (usually a concentric band spaced radially away from both the center of the pad and the perimeter of the pad), the pad conditioning needs to be performed proportionate to the pad surface wear. Moreover, it is desirable to condition a pad in situ and in real-time to avoid costly down-time associated with conditioning processes that stop the polishing of the wafer. Thus, it would be desirable to develop a device for selectively conditioning areas on the pad that require conditioning in situ and in real-time.